Transcription Factor 7-Like 2 Regulates β-Cell Survival and Function in Human Pancreatic Islets

Abstract

OBJECTIVE-Type 2 diabetes is characterized by impaired insulin secretion in response to increased metabolic demand. This defect in ␤-cell compensation seems to result from the interplay between environmental factors and genetic predisposition. Genome-wide association studies reveal that common variants in transcription factor 7-like 2 (TCF7L2) are associated with increased risk of type 2 diabetes. The aim of the present study was to establish whether TCF7L2 plays a role in ␤-cell function and/or survival. RESEARCH DESIGN AND METHODS-To investigate the effects of TCFL7L2 depletion, isolated islets were exposed to TCF7L2 small interfering RNA (siRNA) versus scrambled siRNA, and ␤-cell survival and function were examined. For TCF7L2 overexpression, islets were cultured in glucose concentrations of 5.5-33.3 mmol/l and the cytokine mix interleukin-1␤/␥-interferon with or without overexpression of TCF7L2. Subsequently, glucose-stimulated insulin secretion (GSIS), ␤-cell apoptosis [by transferase-mediated dUTP nick-end labeling assay and Western blotting for poly(ADP-ribose) polymerase and Caspase-3 cleavage], and ␤-cell proliferation (by Ki67 immunostaining) were analyzed. RESULTS-DepletingTCF7L2 by siRNA resulted in a 5.1-fold increase in ␤-cell apoptosis, 2.2-fold decrease in ␤-cell proliferation (P Ͻ 0.001), and 2.6-fold decrease in GSIS (P Ͻ 0.01) in human islets. Similarly, loss of TCF7L2 resulted in impaired ␤-cell function in mouse islets. In contrast, overexpression of TCF7L2 protected islets from glucose and cytokine-induced apoptosis and impaired function. CONCLUSIONS-TCF7L2is required for maintaining GSIS and ␤-cell survival. Changes in the level of active TCF7L2 in ␤-cells from carriers of at-risk allele may be the reason for defective insulin secretion and progression of type 2 diabetes. Diabetes 57: [645][646][647][648][649][650][651][652][653] 2008

Translating TCF7L2: from gene to function

“…Reducing TCF7L2 protein production using siRNA led to a small reduction in glucose-induced insulin secretion and a much greater reduction in GLP1-induced secretion [12,13]. In the first study [12], knockdown of TCF7L2 in human islets decreased beta cell proliferation, and overexpression of TCF7L2 had a protective effect against glucose-and cytokine-induced apoptosis, suggesting a role in regulation of beta cell mass. However, the second study points to a functional defect [11].…”

“…• Decreased beta cell mass Decreased beta cell proliferation following TCF7L2 knockdown [12] Decreased 'maximal' insulin secretion-response to arginine at 28 mmol/l glucose [10] • Impaired insulin processing or release Altered SLC30A8, Munc18-1 and Stx1A expression in mice with Tcf7l2 knockdown [13] Increased proinsulin:insulin ratio in rs7903146 T allele carriers [9,17] Decreased 'maximal' insulin secretion-response to arginine at 28 mmol/l glucose [10] • Impaired GLP1 signalling in beta cells [9,11] Response to GLP1 impaired to greater extent than that to i.v. glucose [10] • Decreased glucagon secretion reducing beta cell insulin secretion [9] Needs replicating…”

“…What remains uncertain is whether the striking incretin effect reflects a GLP1/GIP-specific effect, or whether this is a generalised defect in insulin processing and secretion. In favour of the latter, TCF7L2 expression affects the direct effect of glucose both in vitro [12,13] and in vivo [16]; there is an increase in the proinsulin:insulin ratio in carriers of the risk allele [17], suggesting a defect in insulin processing; and the 'maximum' insulin secretory capacity, as assessed by the response to arginine at a constant plasma glucose concentration of 28 mmol/l, is reduced in risk allele carriers with impaired glucose tolerance or type 2 diabetes [10]. It would be interesting to see how the insulin secretory response to tolbutamide is affected in islets with reduced TCF7L2 expression, or, indeed, in humans carrying the TCF7L2 risk allele, as this may help further address this question.…”

Wnt4 antagonises Wnt3a mediated increases in growth and glucose stimulated insulin secretion in the pancreatic beta-cell line, INS-1

“…Subsequent studies implicated β-cell dysfunction as the underlying cause of diabetes in patients with the TCF7L2 risk variant [2][3][4][5] and have suggested that Wnt signalling plays an important role in both pancreatic development and in the function of mature pancreatic islets [6]. The activation of the canonical Wnt pathway, by ligands such as Wnt3a, has been of particular interest as it can increase β-cell proliferation, decrease apoptosis and improve glucose stimulated insulin secretion [6][7][8][9][10][11][12]. However there is now increasing evidence that non-canonical Wnt ligands such as Wnt4 can antagonise the signalling mediated by Wnt3a and may play an equally important role in β-cell function [13][14][15].…”

Wingless-type MMTV integration site family (WNT) signalling in pancreatic beta cells—more complex than expected

“…In line with this, molecular studies indicate co-activation of the gene encoding glucokinase-a major regulator of glucose-sensing in beta-cells-by β-catenin [3]. In addition, knocking down the gene encoding TCF7L2 in adult murine and human islets in vitro reduces beta cell proliferation and insulin secretion [7], emphasising the importance of this transcription factor for beta cell function. Of note, TCF7L2 is a classic target for β-catenin.…”